U.S. patent application number 14/553580 was filed with the patent office on 2015-05-28 for system and method for radio frequency carrier aggregation.
The applicant listed for this patent is PlusN, LLC. Invention is credited to John David Terry.
Application Number | 20150146805 14/553580 |
Document ID | / |
Family ID | 53182651 |
Filed Date | 2015-05-28 |
United States Patent
Application |
20150146805 |
Kind Code |
A1 |
Terry; John David |
May 28, 2015 |
SYSTEM AND METHOD FOR RADIO FREQUENCY CARRIER AGGREGATION
Abstract
A system and method for aggregation of a plurality of wireless
communication signals in a common radio frequency transmitter. A
multiple subchannel multiplexed (MSM) signal within a frequency
band, and having a communication protocol with pilot signals
estimating a channel state and efficient signal demodulation with a
mobile receiver is combined with another signal. An automated
digital processor at the transmitter modifies the MSM to meet at
least one fitness criterion of the combined signal, selected from
among alternatives that will maintain good reception of all
transmitted signals within system protocols, without requiring
transmitting additional side information specifying the
modification. The range of alternates include modifications that
disrupt the pilot signals. The fitness criterion may include
minimizing the peak-to-average power ratio of the combination, and
a signal modification may comprise a cyclic permutation of a
portion of the time-domain representation of a signal, while
maintaining compatibility with prior pilot signals.
Inventors: |
Terry; John David;
(Annandale, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PlusN, LLC |
Elmsford |
NY |
US |
|
|
Family ID: |
53182651 |
Appl. No.: |
14/553580 |
Filed: |
November 25, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61909252 |
Nov 26, 2013 |
|
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Current U.S.
Class: |
375/260 ;
375/340; 375/349 |
Current CPC
Class: |
H04L 45/24 20130101;
H04L 27/2607 20130101; H04L 27/2657 20130101; H04J 11/00 20130101;
H04L 27/2655 20130101; H04L 27/367 20130101; H04L 27/2624 20130101;
H04L 5/001 20130101; H04L 27/2618 20130101; H04L 25/0224 20130101;
H04L 27/2621 20130101; H04L 27/2649 20130101; H04L 25/0242
20130101; H04L 27/2626 20130101; H04L 5/0048 20130101; H04L 5/0053
20130101; H04L 5/0007 20130101; H04L 25/0204 20130101; H04L 27/2614
20130101; H04L 25/022 20130101; H04W 84/12 20130101 |
Class at
Publication: |
375/260 ;
375/349; 375/340 |
International
Class: |
H04L 25/02 20060101
H04L025/02; H04L 27/26 20060101 H04L027/26; H04L 5/00 20060101
H04L005/00 |
Claims
1. A method for controlling a combined waveform, representing a
combination of signals, the signals comprising at least one
multiple subchannel multiplexed signal having information modulated
in respective subchannels, with a modulated signal, comprising:
receiving information to be communicated from a transmitter to a
receiver through the at least one multiple subchannel multiplexed
signal according to a predetermined protocol, the multiple
subchannel multiplexed signal comprising pilot signals, within at
least one subchannel for at least a portion of time, having
predefined characteristics sufficiently independent of the
information to be communicated, to permit receiver prediction of a
communication channel state with respect to varying communication
channel conditions; storing a model of the receiver with respect to
a combination of the multiple subchannel multiplexed signal with
the modulated signal in a memory, the model being for predicting a
receiver ability to demodulate the information and a receiver
ability to predict the channel state, over a range of at least one
parameter representing available alterations in the state of the
combination; and defining, with an automated processor, the
combination signals which is predicted to permit sufficient
receiver estimation of the channel state to demodulate the
information from respective subchannels and which further meets at
least one fitness criterion distinct from the receiver estimation
of the channel state and being dependent on both the multiple
subchannel multiplexed signal and the modulated signal, with
respect to at least two different values for the at least one
parameter.
2. The method according to claim 1, further comprising forming a
plurality of sets of combined waveforms, each combined waveform
representing a respective combination of the multiple subchannel
multiplexed signal with the modulated signal at a respective value
of the at least one parameter, and said defining comprises
selecting a respective defined waveform which meets the at least
one fitness criterion.
3. The method according to claim 2, wherein the at least one
fitness criterion comprises a peak to average power ratio.
4. The method according to claim 1, wherein the modulated signal is
modulated with second information, wherein the automated processor
further defines the combination of the multiple subchannel
multiplexed signal with the modulated signal in a manner which is
predicted to permit demodulation of the second information from the
modulated signal.
5. The method according to claim 1, wherein a first combination and
a second combination based on different values of the at least one
parameter differ with respect to a relative timing of a modulation
of frequency components of a first signal with respect to a second
signal.
6. The method according to claim 1, wherein a first combination and
a second combination, having different values of the parameter,
differ with respect to a relative phase of frequency components of
a respective signal.
7. The method according to claim 1, wherein the at least one
multiple subchannel multiplexed signal comprises at least one
orthogonal frequency division multiplexed signal, and the modulated
signal comprises an orthogonal frequency division multiplexed
signal.
8. The method according to claim 1, wherein the at least one
multiple subchannel multiplexed signal is an orthogonal frequency
division multiplexed stream which is compatible with at least one
protocol selected from the group consisting of an IEEE 802.11
protocol, an IEEE 802.16 protocol, a 3GPP-LTE downlink protocol,
LTE-Advanced protocol, a DAB protocol and a DVB protocol, wherein a
receiver compliant with the at least one protocol can demodulate
the at least two respectively different combinations without
requiring additional information to be transmitted outside of said
protocol.
9. The method according to claim 1, wherein the at least two
different values of the at least one parameter correspond to
signals that differ respectively in a cyclic time shift in a
modulation sequence.
10. The method according to claim 1, wherein meeting the fitness
criterion comprises analyzing with respect to a predicted error
rate of a reference receiver design for at least one of the
signals.
11. The method according to claim 1, wherein the defined
combination of signals is communicated as a digital representation
of an intermediate frequency representation.
12. The method according to claim 1, wherein the predistorting
compensates for at least a portion of one or more of an analog
non-linearity, a transmission channel impairment, and a receiver
characteristic of an analog radio communication system
communicating using the combination of signals.
13. The method according to claim 1, wherein each of the at least
two signals comprises an orthogonal frequency domain multiplexed
signal having a cyclic prefix, the at least one parameter comprises
a cyclic time shift, and said defining comprises defining at least
two alternate representations which differ in a respective cyclic
time shift.
14. The method according to claim 1, wherein each of the at least
two signals is received as an orthogonal frequency division
multiplexed signal conforming to a communications protocol, at
least one of the signals being modified to generate the at least
two alternate representations differing according to the at least
one parameter, and the at least one fitness criterion comprises a
peak to average power ratio of the combined signal, wherein said
defining selects a combination representing a lowest peak to
average power ratio.
15. The method according to claim 1, wherein the model of the
receiver incorporates prior pilot signals in defining acceptable
values of the at least one parameter that are selected for further
evaluation according to the at least one fitness criterion.
16. The method according to claim 1, wherein the model of the
receiver uses extrapolation of prior pilot signals to generate a
reference signal during time periods when a current pilot signal is
not available.
17. The method according to claim 1, wherein an evaluation of the
fitness criterion for the combination is implemented in parallel
for the different values of the parameter on a programmable gate
array.
18. The method according to claim 1, wherein each of the at least
one multiple subchannel multiplexed signal and the modulated signal
is received as an orthogonal frequency division multiplexed signal
conforming to a communications protocol, at least one of the
signals is modified to generate the at least two alternate
representations, each of which can be demodulated by a receiver
compatible with the protocol without requiring receipt of
additional information outside of the communications protocol, and
the at least one criterion comprises a peak to average power ratio
of the combined signal.
19. The method according to claim 1, wherein: the multiple
subchannel multiplexed signal is an orthogonal frequency
multiplexed signal having a plurality of subcarriers at different
frequencies which concurrently communicate the information; the
multiple subchannel multiplexed signal and the modulated signal
each being within a different communication frequency channel and
being processed as a combined analog signal in at least one analog
signal processing component having a non-linear distortion; the
multiple subchannel multiplexed signal being according to a
predetermined protocol which selectively inserts the pilot signals
in a plurality of the subcarriers at different times and at
different frequencies to estimate the channel state; the at least
one parameter comprises a cyclic shift of digital data representing
the multiple subchannel multiplexed signal, and wherein the model
of the receiver predicts an ability of a receiver which complies
with an OFDM protocol to at least detect the pilot and estimate the
channel state subject to at least two different cyclic shifts; and
the at least one fitness criterion is dependent on the non-linear
distortion of the combination of the multiple subchannel
multiplexed signal with the modulated signal in the analog signal
processing component.
20. An apparatus for controlling a combined waveform, representing
a combination of at least two signals, each having a plurality of
signal components and conveying information, comprising: an input
port configured to receive information defining the at least two
signals; an automated processor configured to: transform a first of
the signals into at least two representations of the conveyed
information within a range of transformations, along with pilot
signal information which varies in frequency and which is
selectively communicated over time, to permit a receiver to
estimate a channel state, having prohibited combinations of
transformation parameters and pilot signal information, combine a
transformed representation of a first of the signals with a second
of the signals, to define at least two alternate combinations
representing the conveyed information, and select one combination
which meets a predetermined criterion and which permits the
receiver to at least estimate the channel state; and an output port
configured to output information representing a respective combined
waveform comprising the selected one combination of the transformed
representation which meets the predetermined criterion.
21. The apparatus according to claim 20, wherein a first of the at
least two representations and a second of the at least two
representations differ with respect to at least one of (a) a
relative timing of a modulation of frequency components of a first
signal with respect to a second signal and (b) a relative phase of
the frequency components of the first signal with respect to the
second signal, and the at least one criterion comprises a peak to
average power ratio (PAPR).
22. The apparatus according to claim 20, wherein at least one
signal is an orthogonal frequency division multiplexed stream which
is compatible with at least one protocol selected from the group
consisting of an IEEE 802.11 protocol, an IEEE 802.16 protocol, a
3GPP-LTE downlink protocol, a DAB protocol and a DVB protocol,
wherein a receiver compliant with the at least one protocol can
demodulate the at least two respectively different combinations of
without requiring additional information to be transmitted outside
of said protocol.
23. The apparatus according to claim 20, wherein the at least two
representations differ respectively in a cyclic time shift in a
modulation sequence, and the at least one criterion comprises a
peak to average power ratio, wherein an alternate representation
which results in a lowest peak to average power ratio is selected
for combination.
24. The apparatus according to claim 20, wherein the automated
processor is configured to predistort, to compensate for at least a
portion of one or more of an analog non-linearity, a transmission
channel impairment, and a receiver characteristic of an analog
radio communication system communicating using the selected at
least one combination.
25. The apparatus according to claim 20, wherein said automated
processor is further configured to predistort at least one of an
intermediate frequency and a radio frequency representation of the
selected at least one combination.
26. A system for controlling a combined waveform, representing a
combination of at least one multiple subchannel multiplexed signal
according to a predetermined protocol to be communicated from a
transmitter to a receiver with a modulated signal, the multiple
subchannel multiplexed signal comprising pilot signals, within at
least one subchannel for at least a portion of time, having
predefined characteristics sufficiently independent of information
to be communicated, to permit receiver prediction of a
communication channel state with respect to varying communication
channel conditions, comprising: a model of a receiver stored in a
memory, dependent on a combination of the multiple subchannel
multiplexed signal with the modulated signal in a memory, for
predicting a receiver ability to demodulate information and a
receiver ability to predict the channel state, over a range of at
least one parameter representing an alteration in the state of the
combination; and an automated processor configured to define the
combination of the multiple subchannel multiplexed signal with the
modulated signal which is predicted to permit sufficient receiver
estimation of the channel state to demodulate the information and
which further meets at least one fitness criterion distinct from
the receiver estimation of the channel state and being dependent on
both the multiple subchannel multiplexed signal and the modulated
signal, with respect to at least two different values for the at
least one parameter.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority from Provisional
Application Ser. No. 61/909,252, flied Nov. 26, 2013, the entirety
of which is expressly incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to the field of wireless
communications of radio-frequency signals. More specifically, it
relates to controlling a combined signal, for example to reduce its
peak to average power ratio or an inferred error at a receiver.
BACKGROUND OF THE INVENTION
[0003] A common signal format for mobile wireless communications is
orthogonal frequency-domain multiplexing (OFDM) (see, for example,
en.wikipedia.org/Orthogonal_frequency-division_multiplexing), and
closely related formats such as orthogonal frequency-domain
multiple access (OFDMA). For a signal conveyed on an OFDM channel,
this is characterized in the frequency domain by a bundle of narrow
adjacent subchannels, and in the time domain by a relatively slow
series of OFDM symbols each with a time T, each separated by a
guard interval .DELTA.T (see FIG. 1B). Within the guard interval
before each symbol is a cyclic prefix (CP), comprised of the same
signal in the symbol period, cyclically shifted in time. This CP is
designed to reduce the sensitivity of the received signal to
precise time synchronization in the presence of multipath, i.e.,
radio-frequency signals reflecting from large objects in the
terrain such as tall buildings, hills, etc. If a given symbol is
received with a slight time delay (less than .DELTA.T), it will
still be received without error. In addition to the data symbols
associated with the OFDM "payload", there is also typically a
"preamble" signal that establishes timing and other standards. The
preamble may have its own CP, not shown in FIG. 1B.
[0004] In addition to the preamble, a set of pilot symbols (also
called training symbols) are typically interleaved (in time and
frequency) among the data symbols in the payload. These pilot
symbols are used together with the preamble for further refinement
of timing, channel estimation, and signal equalization at the
receiver. The particular placement of pilot symbols in time and
frequency within the payload may differ among various OFDM standard
protocols. A typical example of the placement of pilot symbols in
the time-frequency resource grid is shown in FIG. 2 for a protocol
known as "Long-Term Evolution" (LTE). (See, for example,
www.mathworks.com/help/lte/ug/channel-estimation.html for further
information.) Here pilot symbols are located at four different
frequencies, with a pattern that repeats every eight symbol
periods. This enables the receiver to obtain information on
time-varying channel estimation across the entire resource grid,
using interpolation of the various pilot symbols.
[0005] In OFDM, the sub-carrier frequencies are chosen so that the
sub-carriers are orthogonal to each other, meaning that cross-talk
between the sub-channels is eliminated and inter-sub-carrier guard
bands are not required. This greatly simplifies the design of both
the transmitter and the receiver; unlike conventional FDM, a
separate filter for each sub-channel is not required. The
orthogonality requires that the sub-carrier spacing is
.DELTA.f=k/(T.sub.U) Hertz, where T.sub.U seconds is the useful
symbol duration (the receiver side window size), and k is a
positive integer, typically equal to 1. Therefore, with N
sub-carriers, the total passband bandwidth will be
B.apprxeq.N.DELTA.f (Hz). The orthogonality also allows high
spectral efficiency, with a total symbol rate near the Nyquist
rate. Almost the whole available frequency band can be utilized.
OFDM generally has a nearly "white" spectrum, giving it benign
electromagnetic interference properties with respect to other
co-channel users.
[0006] When two OFDM signals are combined, the result is in general
a non-orthogonal signal. While a receiver limited to the band of a
single OFDM signal would be generally unaffected by the
out-of-channel signals, when such signals pass through a common
power amplifier, there is an interaction, due to the inherent
nonlinearities of the analog system components.
[0007] OFDM requires very accurate frequency synchronization
between the receiver and the transmitter; with frequency deviation
the sub-carriers will no longer be orthogonal, causing intercarrier
interference (ICI), i.e. cross-talk between the sub-carriers.
Frequency offsets are typically caused by mismatched transmitter
and receiver oscillators, or by Doppler shift due to movement.
While Doppler shift alone may be compensated for by the receiver,
the situation is worsened when combined with multipath, as
reflections will appear at various frequency offsets, which is much
harder to correct.
[0008] The orthogonality allows for efficient modulator and
demodulator implementation using the fast Fourier transform (FFT)
algorithm on the receiver side, and inverse FFT (IFFT) on the
sender side. While the FFT algorithm is relatively efficient, it
has modest computational complexity which may be a limiting
factor.
[0009] One key principle of OFDM is that since low symbol rate
modulation schemes (i.e. where the symbols are relatively long
compared to the channel time characteristics) suffer less from
intersymbol interference caused by multipath propagation, it is
advantageous to transmit a number of low-rate streams in parallel
instead of a single high-rate stream. Since the duration of each
symbol is long, it is feasible to insert a guard interval between
the OFDM symbols, thus eliminating the intersymbol interference.
The guard interval also eliminates the need for a pulse-shaping
filter, and it reduces the sensitivity to time synchronization
problems.
[0010] The cyclic prefix, which is transmitted during the guard
interval, consists of the end of the OFDM symbol copied into the
guard interval, and the guard interval is transmitted followed by
the OFDM symbol. The reason that the guard interval consists of a
copy of the end of the OFDM symbol is so that the receiver will
integrate over an integer number of sinusoid cycles for each of the
multipaths when it performs OFDM demodulation with the FFT.
[0011] The effects of frequency-selective channel conditions, for
example fading caused by multipath propagation, can be considered
as constant (flat) over an OFDM sub-channel if the sub-channel is
sufficiently narrow-banded, i.e. if the number of sub-channels is
sufficiently large. This makes equalization far simpler at the
receiver in OFDM in comparison to conventional single-carrier
modulation. The equalizer only has to multiply each detected
sub-carrier (each Fourier coefficient) by a constant complex
number, or a rarely changed value. Therefore, receivers are
generally tolerant of such modifications of the signal, without
requiring that explicit information be transmitted.
[0012] OFDM is invariably used in conjunction with channel coding
(forward error correction), and almost always uses frequency and/or
time interleaving. Frequency (subcarrier) interleaving increases
resistance to frequency-selective channel conditions such as
fading. For example, when a part of the channel bandwidth is faded,
frequency interleaving ensures that the bit errors that would
result from those subcarriers in the faded part of the bandwidth
are spread out in the bit-stream rather than being concentrated.
Similarly, time interleaving ensures that bits that are originally
close together in the bit-stream are transmitted far apart in time,
thus mitigating against severe fading as would happen when
travelling at high speed. Therefore, similarly to equalization per
se, a receiver is typically tolerant to some degree of
modifications of this type, without increasing the resulting error
rate.
[0013] The OFDM signal is generated from the digital baseband data
by an inverse (fast) Fourier transform (IFFT), which is
computationally complex, and as will be discussed below, generates
a resulting signal having a relatively high peak to average power
ratio (PAPR) for a set including a full range of symbols. This high
PAPR, in turn generally leads to increased acquisition costs and
operating costs for the power amplifier (PA), and typically a
larger non-linear distortion as compared to systems designed for
signals having a lower PAPR. This non-linearity leads, among other
things, to clipping distortion and intermodulation (IM) distortion,
which have the effect of dissipating power, causing out-of-band
interference, and possibly causing in-band interference with a
corresponding increase in bit error rate (BER) at a receiver.
[0014] In a traditional type OFDM transmitter, a signal generator
performs error correction encoding, interleaving, and symbol
mapping on an input information bit sequence to produce
transmission symbols. The transmission symbols are subjected to
serial-to-parallel conversion at the serial-to-parallel (S/P)
converter and converted into multiple parallel signal sequences.
The S/P converted signal is subjected to inverse fast Fourier
transform at the IFFT unit. The signal is further subjected to
parallel-to-serial conversion at the parallel-to-serial (P/S)
converter, and converted into a signal sequence. Then, guard
intervals are added by the guard interval (GI) adding unit. The
formatted signal is then up-converted to a radio frequency,
amplified at the power amplifier, and finally transmitted as an
OFDM signal by a radio antenna.
[0015] On the other end, in a traditional type OFDM receiver, the
radio frequency signal is down-converted to baseband or an
intermediate frequency, and the guard interval is removed from the
received signal at the guard interval removing unit. Then, the
received signal is subjected to serial-to-parallel conversion at
S/P converter, fast Fourier transform at the fast Fourier transform
(FFT) unit, and parallel-to-serial conversion at P/S converter.
Then, the decoded bit sequence is output.
[0016] It is conventional for each OFDM channel to have its own
transmit chain, ending in a power amplifier (PA) and an antenna
element. However, in some cases, one may wish to transmit two or
more separate OFDM channels using the same PA and antenna, as shown
in FIG. 3. This is sometimes called "carrier aggregation". This may
permit a system with additional communications bandwidth on a
limited number of base-station towers. Given the drive for both
additional users and additional data rate, this is highly
desirable. The two channels may be combined at an intermediate
frequency using a two-stage up-conversion process as shown in FIG.
3. Although amplification of real baseband signals is shown in FIG.
3, in general one has complex two-phase signals with in-phase and
quadrature up-conversion (not shown). FIG. 3 also does not show the
boundary between digital and analog signals. The baseband signals
are normally digital, while the RF transmit signal is normally
analog, with digital-to-analog conversion somewhere between these
stages.
[0017] Consider two similar channels, each with average power
P.sub.0 and maximum instantaneous power P.sub.1. This corresponds
to a peak-to-average power ratio PAPR=P.sub.1/P.sub.0, usually
expressed in dB as PAPR[dB]=10 log(P.sub.1/P.sub.0). For the
combined signal, the average power is 2 P.sub.0 (an increase of 3
dB), but the maximum instantaneous power can be as high as 4
P.sub.0, an increase of 6 dB. Thus, PAPR for the combined signal
can increase by as much as 3 dB. This maximum power will occur if
the signals from the two channels happen to have peaks which are in
phase. This may be a rare transient occurrence, but in general the
linear dynamic range of all transmit components must be designed
for this possibility. Nonlinearities will create intermodulation
products, which will degrade the signal and cause it to spread into
undesirable regions of the spectrum. This, in turn, may require
filtering, and in any case will likely reduce the power efficiency
of the system.
[0018] Components with required increases in linear dynamic range
to handle this higher PAPR include digital-to-analog converters,
for example, which must have a larger number of effective bits to
handle a larger dynamic range. But even more important is the power
amplifier (PA), since the PA is generally the largest and most
power-intensive component in the transmitter. While it is sometimes
possible to maintain components with extra dynamic range that is
used only a small fraction of the time, this is wasteful and
inefficient, and to be avoided where possible. An amplifier with a
larger dynamic range typically costs more than one with a lower
dynamic range, and often has a higher quiescent current drain and
lower efficiency for comparable inputs and outputs.
[0019] This problem of the peak-to-average power ratio (PAPR) is a
well-known general problem in OFDM and related waveforms, since
they are constructed of multiple closely-spaced subchannels. There
are a number of classic strategies to reducing the PAPR, which are
addressed in such review articles as "Directions and Recent
Advances in PAPR Reduction Methods", Hanna Bogucka, Proc. 2006 IEEE
International Symposium on Signal Processing and Information
Technology, pp. 821-827, incorporated herein by reference. These
PAPR reduction strategies include amplitude clipping and filtering,
coding, tone reservation, tone injection, active constellation
extension, and multiple signal representation techniques such as
partial transmit sequence (PTS), selective mapping (SLM), and
interleaving. These techniques can achieve significant PAPR
reduction, but at the expense of transmit signal power increase,
bit error rate (BER) increase, data rate loss, increase in
computational complexity, and so on. Further, many of these
techniques require the transmission of additional side-information
(about the signal transformation) together with the signal itself,
in order that the received signal be properly decoded. Such
side-information reduces the generality of the technique,
particularly for a technology where one would like simple mobile
receivers to receive signals from a variety of base-station
transmitters. To the extent compatible, the techniques disclosed in
Bogucka, and otherwise known in the art, can be used in conjunction
with the techniques discussed herein-below.
[0020] Various efforts to solve the PAPR (Peak to Average Power
Ratio) issue in an OFDM transmission scheme, include a frequency
domain interleaving method, a clipping filtering method (See, for
example, X. Li and L. J. Cimini, "Effects of Clipping and Filtering
on the Performance of OFDM", IEEE Commun. Lett., Vol. 2, No. 5, pp.
131-133, May, 1998), a partial transmit sequence (PTS) method (See,
for example, L. J Cimini and N. R. Sollenberger, "Peak-to-Average
Power Ratio Reduction of an OFDM Signal Using Partial Transmit
Sequences", IEEE Commun. Lett., Vol. 4, No. 3, pp. 86-88, March,
2000), and a cyclic shift sequence (CSS) method (See, for example,
G. Hill and M. Faulkner, "Cyclic Shifting and Time Inversion of
Partial Transmit Sequences to Reduce the Peak-to-Average Ratio in
OFDM", PIMRC 2000, Vol. 2, pp. 1256-1259, September 2000). In
addition, to improve the receiving characteristic in OFDM
transmission when a non-linear transmission amplifier is used, a
PTS method using a minimum clipping power loss scheme (MCPLS) is
proposed to minimize the power loss clipped by a transmission
amplifier (See, for example, Xia Lei, Youxi Tang, Shaoqian Li, "A
Minimum Clipping Power Loss Scheme for Mitigating the Clipping
Noise in OFDM", GLOBECOM 2003, IEEE, Vol. 1, pp. 6-9, December
2003). The MCPLS is also applicable to a cyclic shifting sequence
(CSS) method.
[0021] In a partial transmit sequence (PTS) scheme, an appropriate
set of phase rotation values determined for the respective
subcarriers in advance is selected from multiple sets, and the
selected set of phase rotations is used to rotate the phase of each
of the subcarriers before signal modulation in order to reduce the
peak to average power ratio (See, for example, S. H. Muller and J.
B. Huber, "A Novel Peak Power Reduction Scheme for OFDM", Proc. of
PIMRC '97, pp. 1090-1094, 1997; and G. R. Hill, Faulkner, and J.
Singh, "Deducing the Peak-to-Average Power Ratio in OFDM by
Cyclically Shifting Partial Transmit Sequences", Electronics
Letters, Vol. 36, No. 6, 16.sup.th March, 2000).
[0022] When multiple radio signals with different carrier
frequencies are combined for transmission, this combined signal
typically has an increased PAPR, owing to the possibility of
in-phase combining of peaks, requiring a larger power amplifier
(PA) operating at low average efficiency. As taught by U.S. Pat.
No. 8,582,687 (J. D. Terry), expressly incorporated herein by
reference in its entirety, the PAPR for digital combinations of
OFDM channels may be reduced by a Shift-and-Add Algorithm (SAA):
Storing the time-domain OFDM signals for a given symbol period in a
memory buffer, carrying out cyclic time shifts to transform at
least one OFDM signal, and adding the multiple OFDM signals to
obtain at least two alternative combinations. In this way, one can
select the time-shift corresponding to reduced PAPR of the combined
multi-channel signal. This may be applied to signals either at
baseband, or on upconverted signals. Several decibels reduction in
PAPR can be obtained without degrading system performance. No side
information needs to be transmitted to the receiver, provided that
the shifted signal can be demodulated by the receiver without
error. This is shown schematically in FIG. 4.
[0023] Some OFDM protocols may require a pilot symbol every symbol
period, where the pilot symbol may be tracked at the receiver to
recover phase information (see FIG. 5). If the time-shift is
performed on a given OFDM carrier, according to such a protocol,
during a specific symbol period, the pilot symbol will be subject
to the same time-shift, so that the receiver will automatically
track these time-shifts from one symbol period to the next.
However, as indicated in FIG. 2, typical modern OFDM protocols
incorporate a sparser distribution of pilot symbols, with
interpolation at the receiver to generate virtual pilot symbols
(reference signals) for other locations. With such a protocol, an
arbitrary time shift as implemented in the SAA may not be properly
tracked, so that without side information, bit errors may be
generated at the receiver.
[0024] What is needed is a practical method and associated
apparatus for reducing the PAPR of combined OFDM signals in a wide
variety of modern OFDM protocols, in a way that does not degrade
the received signal or require the transmission of
side-information.
[0025] The following patents, each of which are expressly
incorporated herein by reference, relate to peak power ratio
considerations: U.S. Pat. Nos. 7,535,950; 7,499,496; 7,496,028;
7,467,338; 7,463,698; 7,443,904; 7,376,202; 7,376,074; 7,349,817;
7,345,990; 7,342,978; 7,340,006; 7,321,629; 7,315,580; 7,292,639;
7,002,904; 6,925,128; 7,535,950; 7,499,496; 7,496,028; 7,467,338;
7,443,904; 7,376,074; 7,349,817; 7,345,990; 7,342,978; 7,340,006;
7,339,884; 7,321,629; 7,315,580; 7,301,891; 7,292,639; 7,002,904;
6,925,128; 5,302,914; 20100142475; 20100124294; 20100002800;
20090303868; 20090238064; 20090147870; 20090135949; 20090110034;
20090110033; 20090097579; 20090086848; 20090080500; 20090074093;
20090067318; 20090060073; 20090060070; 20090052577; 20090052561;
20090046702; 20090034407; 20090016464; 20090011722; 20090003308;
20080310383; 20080298490; 20080285673; 20080285432; 20080267312;
20080232235; 20080112496; 20080049602; 20080008084; 20070291860;
20070223365; 20070217329; 20070189334; 20070140367; 20070121483;
20070098094; 20070092017; 20070089015; 20070076588; 20070019537;
20060268672; 20060247898; 20060245346; 20060215732; 20060126748;
20060120269; 20060120268; 20060115010; 20060098747; 20060078066;
20050270968; 20050265468; 20050238110; 20050100108; 20050089116;
and 20050089109.
[0026] The following patents, each of which is expressly
incorporated herein by reference, relate to one or more topics in
wireless radio-frequency communication systems: U.S. Pat. Nos.
8,130,867; 8,111,787; 8,204,141; 7,646,700; 8,520,494; 20110135016;
20100008432; 20120039252; 20130156125; 20130121432; 20120328045;
2013028294; 2012275393; 20110280169; 2013001474; 20120093088;
2012224659; 20110261676; WO2009089753; WO2013015606, 20100098139;
20130114761; W02010077118A2.
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SUMMARY OF THE INVENTION
[0160] The present invention extends and generalizes the prior art
of Terry (US U.S. Pat. No. 8,582,687) in the carrier aggregation of
two or more OFDM signals in different frequency bands. For a
preferred embodiment, consider a first and a second OFDM signal to
be combined and transmitted, where in a given symbol period, the
candidate signal transformations of the first OFDM signal are
restricted to those that can be demodulated by a receiver without
side information (see FIG. 6). For example, the digital processor
at the transmitter (the "transmit processor") can use the pilot
symbols previously transmitted for the first signal to interpolate
the same virtual pilot symbols (reference signals) that are also
generated by the receiver. Then the transmit processor can select
at least two versions of the first OFDM signal that are compatible
with the receive protocol. Each of these at least two OFDM signal
versions can be combined with the second OFDM signal at a different
frequency (carrier aggregation) as per the prior art Terry
algorithm, and the PAPR (or other figure of merit) for the combined
signal can be evaluated. In this way, the optimization can be
restricted to appropriate signal candidates, without wasting
computational resources on unacceptable alternatives.
[0161] This method can be further generalized beyond OFDM for any
present or future communications system based on multiple
subchannels, which might be labeled MSM (multiple subchannel
multiplexing). Typically, such subchannels are orthogonal, that is,
not subject to intersymbol interference across channels, but this
is not an absolute requirement, since in various instances, such
interference can be resolved, and in any case when subject to
significant Doppler shifts, true orthogonality may be lost. On the
other hand, signals may be defined absent strict orthogonality, but
be received as orthogonal signals, for example, due to Doppler
shifts.
[0162] The subchannels are typically frequency channels, such that
the subchannels represent a series of frequency assignments within
a channel which occupies a range of frequencies. In some cases,
however, the subchannels may correspond to other assignments. For
example, a typical scheme for generating subchannels in an
orthogonal frequency division multiplexed (OFDM) signal is to
modulate a signal by performing an inverse fast Fourier transform
(IFFT) at the transmitter to generate the orthogonal frequency
subcarriers, and perform a fast Fourier transform (FFT) at the
receiver to demodulate the information from the subcarriers.
Typically, a sparse selection of the subcarriers over time and over
frequency communicate pilot signals, which permit calibration of
the receiver to account for channel conditions. One issue addressed
by the present technology is that a time shift of the entire OFDM
signal can lead, under some conditions of the time shift and the
frequency subchannel placement of the pilot signal, to a failure of
an ability to properly receive the pilot signal, and therefore
represents an invalid combination of pilot frequency and cyclic
shift. Therefore according to one aspect of the technology, a
proposed cyclic shift of a multiple subchannel modulated signal is
tested against a model of the receiver and/or the channel
conditions and receiver, to ensure compatibility with successful
receipt of the pilot signal(s) communicated within a symbol
period.
[0163] The technology can be extended to non-frequency subchannel
assignments, for example when the transform used a the transmitter
is a different transform, for example, an inverse wavelet transform
with a corresponding wavelet transform performed at the receiver.
See, each of which is expressly incorporated herein by reference in
its entirety: [0164] Rohit Bodhe et al., "Design Of Simulink Model
For OFDM and Comparison of FFT-OFDM and DWT-OFDM", International
Journal of Engineering Science and Technology (IJEST) Vol. 4 No.
05, May 2012 pp. 1914-1924; 1. Communication Systems, 4th edition,
Simon Haykin, John Wiley and Sons, Inc.; [0165] C. V. Bouwel, et.
al, Wavelet Packet Based Multicarrier Modulation, IEEE
Communications and Vehicular Technology, SCVT 200, pp. 131-138,
2000; [0166] B. G. Nagesh, H. Nikookar, Wavelet Based OFDM for
Wireless Channels, IEEE Vehicular Technology Conference, Vol. 1,
pp. 688-691, 2001; LI Wiehua, et. al, Bi-orthogonal Wavelet Packet
based Multicarrier modulation; [0167] Haixia Zhang, et. al,
Research of DFT-OFDM and DWT-OFDM on Different Transmission
Scenarios. Proceeding of the second international conference on
Information Technology for Application (ICITA 2004); [0168] B. G.
Negash and H. Nikookar, "Wavelet based OFDM for wireless channels,"
Vehicular Technology Conference, 2001; [0169] A. N. Akansu and L.
Xueming, "A comparative performance evaluation of DMT (OFDM) and
DWMT (DSBMT) based DSL communications systems for single and
multitone interference," Proceedings of the IEEE International
Conference on Acoustics, Speech and Signal Processing, 1998; [0170]
Khaizuran Abdullah, "Performance of Fourier-Based and Wavelet-Based
OFDM for DVB-T Systems", 2007 Australasian Telecommunication
Networks and Applications Conference December 2nd-5th 2007,
Christchurch, New Zealand pp. 475-479 (2007); [0171] Rohit Bodhe et
al, "Performance Comparison of FFT and DWT based OFDM and Selection
of Mother Wavelet for OFDM" (IJCSIT) International Journal of
Computer Science and Information Technologies, Vol. 3 (3), 2012,
pp. 3993-3997.
[0172] Similarly to OFDM, any such system may employ at least one
pilot signal transmitted to the receiver, in order to calibrate the
state of the communication channel. Since the channel state will
vary with time and across subchannels (particularly in a mobile
system with multipath and interference), the pilot signal may be
interleaved among the signal data representation present in the
subchannels over time. Typically, a small portion of the
subchannels over time and over a range of different frequencies,
will be allocated to transmitting pilot signals. This will permit
the receiver to track the changing communication channel to
minimize the error rate of received data. An adaptive system may be
provided which alters the insertion of pilot signals interspersed
in the data communication dependent on actual error rates and
channel conditions. Thus, in a channel where the error rate is low,
fewer pilot signals may be communicated, permitting higher peak
data rates. Likewise, under different types of noise conditions,
the pilot signals, which are provided to address changes in channel
conditions, can be traded off against error correction signals,
which address noise conditions.
[0173] Furthermore, the communication protocol should allow the
received data representation to vary along at least one parameter
(in addition to amplitude), in order to reflect the varying channel
environment. A key aspect is the recognition that the transmitted
signal may also incorporate allowable variation in this at least
one parameter. In order to accurately determine the acceptable
range of the allowable variation, the transmit processor may
simulate how the receive processor for the given signal will make
use of the current and prior pilot signals. If two or more signals
or bands are combined (carrier aggregation), the degree of freedom
associated with this allowed variation of the at least one
parameter may be exploited to optimize a separate fitness criterion
or figure of merit, such as a peak-to-average power ratio (PAPR) or
a bit-error ratio (BER) that may vary with this degree of freedom.
In the preferred embodiment described below, the degree of freedom
is cyclic shifting of the signal (which would correspond to
variation in the physical path length), but other transformations
may also be possible, such as frequency shifting (emulating a
Doppler shift), phase shifting, synthetic multipath (time delayed
replica), and deviation from orthogonality for subcarriers.
Furthermore, the description of the preferred embodiment in no way
limits the scope of the invention. In order to identify appropriate
signal transformation candidates, a preferred embodiment of the
present invention builds on a set of digital signal processing
techniques known in the prior art as "codebook transmission" or
"codebook pre-weighting" or "precoding". Codebook transmission
derives its origin from cryptography. A codebook contains a lookup
table for coding and decoding; each word or phrase has one or more
strings which replace it. More recently, the term precoding has
been used in conjunction with closed loop beamforming techniques in
multi-antenna wireless communication systems, where channel state
information is sent to the transmitting device from the receiving
device to optimize the transmission for the current state of the
channel. See, for example, the Wikipedia entry on "Precoding":
en.wikipedia.org/wiki/Precoding. It should not be understood here
that the present method for RF carrier aggregation relies on
closed-loop or multi-antenna systems, but rather that similar
mathematical techniques are applied. However, this similarity also
enables straightforward integration of efficient carrier
aggregation with multi-antenna and closed-loop communication
systems.
[0174] Codebook techniques can generate a lookup table of channel
responses corresponding to at least two different transformations
of the pilot symbols for specific OFDM protocols, and corresponding
allowable channel responses for those locations in the resource
grid without any pilot symbols. This is indicated in the
generalized flowchart shown in FIG. 7. By allowable, we mean that
process steps depicted in FIG. 8 and FIG. 9 allow recovery of the
OFDM data symbols with minimal or no degradation at the receiver
beyond the effects due to the physical channel, and not requiring
any additional side information.
[0175] A preferred embodiment of the present system and method
(shown in FIG. 10 and FIG. 11) seeks to control the PAPR by storing
the time-domain OFDM signals for a given symbol period in a memory
buffer, and carrying out cyclic time shifts of at least one of the
OFDM signals, in order to select the time-shift corresponding to a
desired PAPR of the combined multi-channel signal. In most cases,
it would be desired to reduce the PAPR to a minimum, but this is
not a limitation of the technique, and the selected time-shift may
be based on other criteria.
[0176] It is noted that each of the OFDM signals may be
preprocessed in accordance with known schemes, and thus each may
have been themselves processed to reduce an intrinsic PAPR, though
preferably any preprocessing of the signals is coordinated with the
processing of the combined signals to achieve an optimum cost and
benefit. For example, where two separate signals are to be
combined, each having a high PAPR, a resulting signal of reduced
PAPR can be achieved if the peaks add out of phase, and thus
cancel. Therefore, initial uncoordinated efforts to modify the
input OFDM signals may have limited benefit.
[0177] It is further noted that the present system seeks to combine
independently formatted OFDM signals, which are generally targeted
to different receivers or sets of receivers, and these sets are
typically not coordinated with each other. For example, in a
cellular transceiver system, a base station may serve hundreds or
thousands of cell phones, each phone monitoring a single OFDM
broadcast channel, with the base station servicing multiple OFDM
channels. It is particularly noted that each set of OFDM
subcarriers is orthogonal, but the separate OFDM signals, and their
subcarriers, are generally not orthogonal with each other. The OFDM
signals may be in channels which are adjacent or displaced, and
therefore a relative phase change between OFDM signals can occur
during a single symbol period. Therefore, the PAPR must be
considered over the entire symbol period.
[0178] Indeed, according to another embodiment of the method, it is
not the PAPR of the signal which is analyzed for optimization, but
rather an inferred error at the receiver. Thus, if the PAPR of the
composite signal is high for only a small portion of a symbol
period, such that the PA distorts or clips the signal at that time,
but at most other times the combined signals are well within
specification, the result may be an acceptable transmission which
would likely result in a low error probability. Indeed, in some
cases, the error probability may be lower than for signals with a
lower absolute peak. Therefore, by employing a model of a receiver,
which itself may include margins for specific communication channel
impairments to specific receivers, and Doppler shifts (which may be
determined, for example by analyzing return path characteristics),
or over a range of possible variation, as part of the transmitter
signal processing path, better performance may be available than by
simply minimizing the PAPR.
[0179] The receiver model seeks to implement the critical functions
of an idealized receiver compliant with the communication protocol,
as well as optionally a channel conditions model or range of
possible impairment condition models. In the case of an OFDM
receiver, the received signal is demodulated, e.g., to baseband,
and an FFT applied to separate subbands into frequency bins. In
some timeframes, and some subbands, pilot signals are inserted
instead of data, according to a predetermined protocol. If a small
number of pilot signals are to be extracted from the OFDM signal, a
Goertzel Algorithm may also be used. The receiver knows where these
pilot signals are to be found, and analyzes these separately for
various distortions which indicate channel conditions. Since
channel conditions change slowly with respect to data frames, the
pilot transmission may be sparse, and some data frames may not
include pilot signals. The pilot signals are typically spread into
different frequency bins, to map the conditions across the entire
channel. The remaining frequency bins are then analyzed to extract
the subband data. The pilot signal may be used to correct the
demodulation of data from the information subbands, i.e., calibrate
frequency bin boundaries in the presence of Doppler shift and the
like.
[0180] When the OFDM signal is cyclically shifted, this appears to
the receiver similar to a time shift (delay). Therefore, the cyclic
shift is permissible to the receiver within the range of its
permissible change in time delay between symbols. The receiver
model therefore maintains in a memory the prior states of the time
shifts, which will control the acceptability of a successive change
in time shift.
[0181] According to the model, if the various pilot signals are
sufficiently corrupted, the data cannot reliably be demodulated
from the OFDM signal, and a packet retransmit, for example, is
requested. In the receiver model at the transmitter according to
the present technology, therefore, the modified OFDM signal, e.g.,
a cyclically shifted representation of the OFDM signal, is analyzed
to ensure that the pilot signals contained in the stream of OFDM
symbols may be properly detected, and therefore would likely be
properly detected by a real receiver through a real communication
channel.
[0182] According to another embodiment, the model is implemented
using at least one lookup table, based on previous applied time
shifts (cyclic shifts). Assuming that the receiver has a specified
margin for time shifts between successive symbols or data blocks,
the lookup table can then estimate the tolerable range of delay to
be added or subtracted from the successive symbol or block, that
will still be within the operating range of the receiver. According
to this model, a demodulation is not per se required. The lookup
table may in some cases be predetermined, but in others it can be
adaptive. For example, different receivers may implement the
standard differently, and thus have different tolerance for
variations in delay. Since the identification of the receiver may
not be available for the transmitter, it may be convenient to test
the range of permissible delays at the beginning of a
communications session, using the occurrence of retransmission
requests to indicate the range of abilities. Note also that packets
from the receiver to the transmitter, such as retransmission
requests, may be analyzed for certain attributes of the channel
conditions, such as relative speed (Doppler shift).
[0183] Another option is to modify the OFDM signal during all or a
portion of the period in a manner which deviates from a standard
protocol, which is, for example an IEEE-802 OFDM standard, WiFi,
WiMax, DAB, DVB, non-orthogonal multi access schemes, 3G, 4G, or 5G
cellular communication, LTE or LTE-Advanced signals, or the like,
but which does not substantively increase a predicted BER (bit
error rate) of a standard or specific receiver. For example, if the
PAPR is high for a small portion a symbol period, such that if
during a portion of the symbol period, one or more subcarriers were
eliminated or modified, the PAPR would be acceptable, and the
signal at the receiver would have sufficient information to be
decoded using a standard receiver without significant increase in
BER, then the transmitter could implement such modifications
without need to transmit side information identifying the
modifications which are necessary for demodulation. Another
possible deviation is, for example, to frequency shift the signal
(which mildly violates the orthogonality criterion), within the
tolerance of a receiver to operate within a range of Doppler shifts
which are equivalent to frequency shifts.
[0184] Consider two OFDM signals that are being combined as in FIG.
10. For simplicity, call Signal 1 (S1) the reference signal, and
Signal 2 (S2) the modified signal. During each OFDM symbol period,
the baseband digital data bits for each signal will be stored in
memory. Assume that the Preamble has been stripped off, but the
Cyclic Prefix CP remains. As indicated in FIG. 10 for one
embodiment of the invention, the bits for the reference signal S1
are stored in a first-in-first-out (FIFO) shift register (SR). The
corresponding bits for the modified signal S2 are stored in a
circular shift register (CSR), so configured that the data
contained can be rotated under program control. The data for both
signals are first up-converted to an intermediate frequency (IF)
and then combined (added), while maintaining digital format at a
sampling frequency increased over the digital data rate. The
combined IF signals are then subjected to a PAPR test, to determine
whether the peak power level is acceptable, or, in other
embodiments, whether other criteria are met. This might correspond,
for example, to a PAPR of 9 dB. If the test is passed, then the
data bits for the combined OFDM symbols are read out, to be
subsequently reassembled into the full OFDM frame and up-converted
to the full RF, for further amplification in the PA and
transmission. According to another embodiment, a combined OFDM
representation of the combined data is itself the source for the
up-conversion.
[0185] More generally, once the parametric transformation (relative
time-shift) to achieve the desired criteria is determined, the
final signal is then formulated dependent on that parameter or a
resulting representation, which may be the digital data bits of the
baseband signal or a converted form thereof; in the latter case,
the system may implement a series of transformations on the data,
some of which are redundant or failed, seeking an acceptable one or
optimum one; once that is found, it may not be necessary to repeat
the series of transformations again. Likewise, the option of
reverting to the original digital data and repeating the determined
series of transformations allows a somewhat different
representation to be formed in the register, for example one which
is simplified or predistorted to allow consideration of analog
component performance issues in the combining test.
[0186] Even more generally, the technique provides that each signal
to be combined is provided with a range of one or more acceptable
parameters, which may vary incrementally, algorithmically,
randomly, or otherwise, and at least a portion of the possible
combinations tested and/or analyzed for conformity with one or more
criteria, and thereafter the combination of OFDM signals
implemented using the selected parameter(s) from a larger set of
available parameters. This parametric variation and testing may be
performed with high speed digital circuits, such as superconducting
logic, in a serial fashion, or slower logic with parallelization as
necessary, though other technologies may be employed as appropriate
and/or necessary, including but not limited to optical computers,
programmable logic arrays, massively parallel computers (e.g.,
graphic processors, such as nVidia Tesla.RTM. GPU, ATI Radeon R66,
R700), and the like. The use of superconducting digital circuits
may be advantageous, for example, where a large number of complex
computations which make significant use of a specialized high speed
processor, such as where a large number of independent receivers
are modeled as part of a transmitter optimization.
[0187] In the preferred embodiment, at any state of the tests over
the parametric range, if the test is not passed, a control signal
is fed back to the register, e.g., CSR, which rotates the data bits
of the modified signal S2. The shifted data is then combined with
the initial stored data from S1 as before, and the PAPR re-tested.
This is repeated until the PAPR test is passed. A similar sequence
of steps is illustrated in FIG. 10, where stripping off the
preamble and reattaching it at the end are explicitly shown. It is
noted that, in some cases, the tests may be applied in parallel,
and therefore a strictly iterative test is not required. This, in
turn, permits use of lower speed testing logic, albeit of higher
complexity. Likewise, at each relative time-shift, a secondary
parameter may also be considered.
[0188] For example, a secondary consideration for optimal combining
may be in-band (non-filterable) intermodulation distortion. Thus,
at each basic parametric variation, the predicted in-band
intermodulation distortion, expressed, for example, as a power
and/or inferred BER, may be calculated. This consideration may be
merged with the PAPR, for example, by imposing a threshold or
optimizing a simple linear combination "cost function", within an
acceptable PAPR range.
[0189] While there may be some delays in this Shift-and-Add process
(SAA), the time for the entire decision algorithm, including all
iterations, must not exceed the expanded symbol time T+.DELTA.T. We
have described a serial decision process in FIGS. 4 and 10. As
discussed above, in some cases, it may be preferable to carry out
parts of this process in parallel, using multiple CSRs with
different shifts and multiple parallel PAPR tests, in order to
complete the process more quickly. This is illustrated in FIG. 11,
which suggests parallel memories (shown here as RAMs), each with an
appropriate time shift, where the minimum PAPR is selected to send
to the RF subsystem. The optimum tradeoff between circuit speed and
complexity will determine the preferred configuration.
[0190] In some situations, the search for an optimum combined
signal requires vast computational resources. In fact, heuristics
may be available to limit the search while still achieving an
acceptable result. In the case of a PAPR optimization, generally
the goal is to test for limited, low probability "worst case"
combinations of symbols. If the raw digital data is available, a
lookup table may be employed to test for bad combinations, which
can then be addressed according to a predetermined modification.
However, for multi-way combinations of complex symbols this lookup
table may be infeasible. On the other hand, the individual OFDM
waveforms may each be searched for peaks, for example 6 dB above
mean, and only these portions of the signal analyzed to determine
whether there is a temporal alignment with the peaks of other OFDM
signals; if the peaks are not temporally synchronized, then a
presumption is made that an unacceptable peak will not result in
the final combined signal. This method makes a presumption that
should be statistically acceptable, that is, that only portions of
an OFDM waveform that are themselves relative peaks will contribute
to large peaks in the combination of OFDM signals. This method
avoids serial testing of sequential parametric variations, and
rather simply avoids worst case superpositions of a binary
threshold condition.
[0191] Although these figures focus on the case of reducing PAPR
for the combination of two OFDM channels, this method is not
limited to two channels. Three or more channels can be optimized by
a similar method of circular time shifts, followed by PAPR tests.
Furthermore, although cyclic shifting is presented as a preferred
embodiment of the proposed method, this is intended to represent a
specific example of a more general signal transformation. Any such
transformation that encodes the same information, and can be
decoded (without error) by the receiver without the transmission of
additional side information, would serve the same purpose. The
identification of such transformations depends on the details of
present and future protocols for wireless signal communication
systems.
[0192] Finally, both the codebook LUT and the signal transformation
may incorporate other digital methods to improve signal fidelity,
such as predistortion (to compensate for power amplifier
nonlinearity) and multi-antenna transmission (MIMO). In this way,
the carrier aggregation method of the present invention can
accommodate new approaches to increase data rate and decrease
noise.
[0193] It is therefore an object to provide a method for
controlling a combined waveform, representing a combination of at
least one multiple subchannel multiplexed (MSM) signal with another
signal, comprising: receiving information to be communicated from a
transmitter to a receiver through the at least one MSM signal
according to a predetermined protocol, the MSM signal comprising
pilot signals, within at least one subchannel for at least a
portion of time, having predefined characteristics sufficiently
independent of the information to be communicated, to permit
receiver prediction of a communication channel state with respect
to varying communication channel conditions; storing a model of the
receiver with respect to prior communications and a combination of
the MSM signal with the other signal in a memory, the model being
for predicting a receiver ability to demodulate the information and
a receiver ability to predict the channel state, over a range of at
least one parameter representing an alteration in the state of the
combination; and defining, with an automated processor, the
combination of the MSM signal with the other signal which is
predicted to permit sufficient receiver estimation of the channel
state to demodulate the information and which further meets at
least one fitness criterion distinct from the receiver estimation
of the channel state and being dependent on both MSM signal and the
other signal, with respect to at least two different values for the
at least one variable parameter.
[0194] It is also an object to provide a system for controlling a
combined waveform, representing a combination of at least one
multiple subchannel multiplexed (MSM) signal according to a
predetermined protocol to be communicated from a transmitter to a
receiver with another signal, the MSM signal comprising pilot
signals, within at least one subchannel for at least a portion of
time, having predefined characteristics sufficiently independent of
the information to be communicated, to permit receiver prediction
of a communication channel state with respect to varying
communication channel conditions, comprising: a model of a receiver
stored in a memory, dependent on prior communications and a
combination of the MSM signal with the other signal in a memory,
for predicting a receiver ability to demodulate information and a
receiver ability to predict the channel state, over a range of at
least one parameter representing an alteration in the state of the
combination; and an automated processor configured to define the
combination of the MSM signal with the other signal which is
predicted to permit sufficient receiver estimation of the channel
state to demodulate the information and which further meets at
least one fitness criterion distinct from the receiver estimation
of the channel state and being dependent on both MSM signal and the
other signal, with respect to at least two different values for the
at least one variable parameter.
[0195] It is a further object to provide a method for controlling a
combined waveform, representing a combination of signals, the
signals comprising at least one multiple subchannel multiplexed
signal having information modulated in respective subchannels, with
a modulated signal, comprising: receiving information to be
communicated from a transmitter to a receiver through the at least
one multiple subchannel multiplexed signal according to a
predetermined protocol, the multiple subchannel multiplexed signal
comprising pilot signals, within at least one subchannel for at
least a portion of time, having predefined characteristics
sufficiently independent of the information to be communicated, to
permit receiver prediction of a communication channel state with
respect to varying communication channel conditions; storing a
model of the receiver with respect to a combination of the multiple
subchannel multiplexed signal with the modulated signal in a
memory, the model being for predicting a receiver ability to
demodulate the information and a receiver ability to predict the
channel state, over a range of at least one parameter representing
available alterations in the state of the combination; and
defining, with an automated processor, the combination signals
which is predicted to permit sufficient receiver estimation of the
channel state to demodulate the information from respective
subchannels and which further meets at least one fitness criterion
distinct from the receiver estimation of the channel state and
being dependent on both the multiple subchannel multiplexed signal
and the modulated signal, with respect to at least two different
values for the at least one parameter.
[0196] It is another object to provide a system for controlling a
combined waveform, representing a combination of signals, the
signals comprising at least one multiple subchannel multiplexed
signal having information modulated in respective subchannels, with
a modulated signal, comprising: an input configured to receive
information to be communicated from a transmitter to a receiver
through the at least one multiple subchannel multiplexed signal
according to a predetermined protocol, the multiple subchannel
multiplexed signal comprising pilot signals, within at least one
subchannel for at least a portion of time, having predefined
characteristics sufficiently independent of the information to be
communicated, to permit receiver prediction of a communication
channel state with respect to varying communication channel
conditions; a memory configured to store a model of the receiver
with respect to an ability to estimate a channel state demodulate
the information, from a combination of the multiple subchannel
multiplexed signal with the modulated signal; at least one
automated processor configured to: define a plurality of alternate
representations of differing combinations of the multiple
subchannel multiplexed signal with the modulated signal, differing
with respect to at least one parameter, wherein the at least one
parameter has a range which includes at least one value that
impairs an ability to estimate a channel state by the receiver; and
select at least one combination of the multiple subchannel
multiplexed signal with the modulated signal which is predicted
based on the model to permit sufficient receiver estimation of the
channel state and to demodulate the information from respective
subchannels with respect to the defined plurality of alternate
representations of differing combinations.
[0197] It is still another object to provide a system for
controlling a combined waveform, representing a combination of at
least one multiple subchannel multiplexed signal according to a
predetermined protocol to be communicated from a transmitter to a
receiver with a modulated signal, the multiple subchannel
multiplexed signal comprising pilot signals, within at least one
subchannel for at least a portion of time, having predefined
characteristics sufficiently independent of information to be
communicated, to permit receiver prediction of a communication
channel state with respect to varying communication channel
conditions, comprising: a model of a receiver stored in a memory,
dependent on a combination of the multiple subchannel multiplexed
signal with the modulated signal in a memory, for predicting a
receiver ability to demodulate information and a receiver ability
to predict the channel state, over a range of at least one
parameter representing an alteration in the state of the
combination; and an automated processor configured to define the
combination of the multiple subchannel multiplexed signal with the
modulated signal which is predicted to permit sufficient receiver
estimation of the channel state to demodulate the information and
which further meets at least one fitness criterion distinct from
the receiver estimation of the channel state and being dependent on
both the multiple subchannel multiplexed signal and the modulated
signal, with respect to at least two different values for the at
least one parameter.
[0198] Another object provides an apparatus for combining a
plurality of signals in a respective plurality of channels, each
signal comprising a set of concurrent phase and/or amplitude
modulated components within a channel, comprising: a processor
configured to: receive information defining each of the plurality
of signals; transform a representation of at least one signal in a
plurality of different ways, each transformed representation
representing the same information, analyze respective combinations
of each transformed representation with information defining at
least one other signal, with respect to at least two different
fitness criteria, and select at least one respective combination as
being fit according to the analysis with respect to the at least
two different fitness criteria; and an output port configured to
present at least one of an identification of the selected at least
one respective combination, the selected at least one respective
combination, and information defining the selected at least one
respective combination, wherein at least one of the criteria
relates to a predicted ability of a receiver to estimate a channel
state of a communication channel, and wherein at least one
transformed representation impairs an ability of the receiver to
successfully estimate the channel state of the communication
channel.
[0199] It is a still further object to provide an apparatus for
combining a plurality of signals in a respective plurality of
channels, each signal comprising a set of phase and/or amplitude
modulated orthogonal frequency components within a channel,
comprising: a processor configured to receive information defining
each of the plurality of signals, being represented as a plurality
of orthogonal frequency multiplexed signal components, transform a
representation of at least one signal in at least two different
ways, each transformed representation representing the same
information, analyze with respect to at least two different fitness
criteria a plurality of different combinations of the plurality of
signals, each of the plurality of representations including
alternate representations of at least one signal subject to at
least two different transformations, and select a combination based
on the analyzing which meets each of the at least two criteria; and
an output port configured to present at least one of an
identification of the selected combination, the selected
combination, and information defining the selected combination,
wherein at least one of the criteria relates to a predicted ability
of a receiver to estimate a channel state of a communication
channel based on pilot sequences within the representation, wherein
at least one transformation of the representation impedes an
ability of the receiver to successfully estimate the channel state
for demodulating the information.
[0200] It is also an object to provide an apparatus for controlling
a combined waveform, representing a combination of at least two
signals, each having a plurality of signal components and conveying
information, comprising: an input port configured to receive
information defining the at least two signals; an automated
processor configured to: transform a first of the signals into at
least two representations of the conveyed information within a
range of transformations, along with pilot signal information which
varies in frequency and which is selectively communicated over
time, to permit a receiver to estimate a channel state, having
prohibited combinations of transformation parameters and pilot
signal information, combine a transformed representation of a first
of the signals with a second of the signals, to define at least two
alternate combinations representing the conveyed information, and
select one combination which meets a predetermined criterion and
which permits the receiver to at least estimate the channel state;
and an output port configured to output information representing a
respective combined waveform comprising the selected one
combination of the transformed representation which meets the
predetermined criterion.
[0201] A plurality of sets of combined waveforms may be formed,
each combined waveform representing a respective combination of the
multiple subchannel multiplexed signal with the modulated signal at
a respective value of the at least one parameter, and said defining
comprises selecting a respective defined waveform which meets the
at least one fitness criterion. The modulated signal is typically
modulated with second information (which may be intelligence or a
pseudorandom noise sequence), and the automated processor may
further define the combination of the multiple subchannel
multiplexed signal with the modulated signal in a manner which is
predicted to permit demodulation of the second information from the
modulated signal.
[0202] The MSM signal may be, for example, an orthogonal frequency
multiplexed signal, but is not so limited. In particular, the
subcarriers need not be orthogonal, and indeed, the subcarriers
need not be distributed according to frequency, though such an
arrangement is presently preferred, especially to the extent that
the receivers are standard OFDM receivers. The MSM signal may also
be a wavelet encoded signal, in which case the discrete wavelet
transform (DWT) and corresponding inverse wavelet transform (IWT)
generally replace the FFT and IFT employed within OFDM
communications. An orthogonality constraint may be relaxed such
that the receiver in the estimated state can demodulate the
information, without strictly meeting the constraint.
[0203] In general, MSM signals are intended to be communicated to,
and received by, mobile receivers, and therefore the communication
protocol provides tolerance to various types of interference and
distortion. For example, time varying multipath, Doppler shifts,
and the like, are tolerable. The present technology can model the
receiver with respect to the tolerance, for example by calculating
a bit error rate, or data throughput rate (dependent on
retransmission of packets and error detection and correction (EDC)
code burden), and optimizing the combined signal.
[0204] The plurality of signals may each comprise orthogonal
frequency division multiplexed signals. A first combination and a
second combination based on different values of the variable
parameter may differ with respect to at least one of (a) a relative
timing of a modulation of the frequency components of a first
signal with respect to a second signal and (b) a relative phase of
the frequency components of the first signal with respect to the
second signal. The at least one criterion may comprise a peak to
average power ratio (PAPR). The selected at least one respective
combination may comprise a combination that is beneath a threshold
peak to average power ratio.
[0205] The at least one fitness criterion may comprise a peak to
average power ratio (PAPR).
[0206] The signals may comprise orthogonal frequency division
multiplexed (OFDM) signals. At least one signal may be an
orthogonal frequency division multiplexed stream which is
compatible with at least one protocol selected from the group of an
IEEE 802.11 protocol, an IEEE 802.16 protocol, a 3GPP-LTE downlink
protocol, LTE-Advanced protocol, a DAB protocol and a DVB protocol,
wherein a receiver compliant with the at least one protocol can
demodulate the at least two respectively different combinations of
without requiring additional information to be transmitted outside
of the protocol.
[0207] The at least two different values of the variable parameter
may correspond to signals that differ respectively in a cyclic time
shift in a modulation sequence.
[0208] The at least two different values of the variable parameter
may correspond to signals that differ respectively in a cyclic time
shift in a modulation sequence, and the at least one fitness
criterion may comprises a peak to average power ratio (PAPR). An
alternate representation which results in a lowest peak to average
power ratio, or a peak to average power ratio below a threshold,
may be selected for combination.
[0209] At least one of an intermediate frequency at a frequency
below about 125 MHz and a radio frequency representation at a
frequency greater than about 500 MHz of the defined combination of
signals may be predistorted.
[0210] The automated processor may comprise superconducting digital
logic circuits. Alternately, the automated processor may comprise a
programmable logic device, programmable logic array, CPLD, RISC
processor, CISC processor, SIMD processor, general purpose graphics
processing unit (GPGPU) or the like, implemented in silicon
technology, superconducting digital logic circuits, or other types
of logic.
[0211] Meeting the fitness criterion may comprise analyzing with
respect to dynamic range of a respective combination, or analyzing
with respect to a predicted error rate of a reference receiver
design for one of the signals, or analyzing with respect to a peak
to average power ratio of the combined waveform and a predicted
error rate of a receiver for one of the signals, or analyzing a
clipping distortion of the combined waveform, for example.
[0212] The combined waveform may be a digital representation that
is sampled at a data rate higher than the corresponding data rates
of any of the component signals or a digital representation of an
intermediate frequency representation of the combined signal, for
example.
[0213] Generating the combined waveform may comprise outputting a
set of parameters for converting a digital baseband signal into the
selected combined signal.
[0214] The method may further comprise predistorting at least one
of an intermediate frequency and a radio frequency representation
of the selected combined signal. The predistorting may compensate
for at least a portion of one or more of an analog non-linearity, a
transmission channel impairment, and a receiver characteristic of
an analog radio communication system communicating using the
selected combined signal. The predistorting may also compensate for
a non-linear distortion of a power amplifier which amplifies the
selected combined signal.
[0215] Each of the at least two signals may comprise an orthogonal
frequency domain multiplexed signal having a cyclic prefix, and
wherein the two alternate representations differ in a respective
cyclic time shift.
[0216] Each of the at least two signals may be received as an
orthogonal frequency division multiplexed signal conforming to a
communications protocol, at least one of the signals may be
modified to generate the at least two alternate representations,
and the at least one fitness criterion may comprise a peak to
average power ratio of the combined signal, wherein the selected
combined signal is a combined signal representing a lowest peak to
average power ratio or a peak to average power ratio below a
predetermined threshold.
[0217] The receiver model may incorporate prior pilot signals in
defining acceptable values of the variable parameter that are
selected for further evaluation according to the fitness
criterion.
[0218] The selection of acceptable values of the variable parameter
may be implemented with the use of an adaptive lookup table memory.
The automated processor may be configured to retrieve values from
the lookup table for the selection of the at least one
combination.
[0219] The receiver model may extrapolate prior pilot signals to
generate a reference signal during time periods when a current
pilot signal is not available.
[0220] A buffer memory may be used to store the input signals until
the preferred combination for transmission is defined. The buffer
memory may comprise at least one shift register.
[0221] The evaluation of the fitness criterion for the combination
may be implemented in parallel for the different values of the
variable parameter.
[0222] The automated processor may comprise a programmable gate
array.
[0223] Each of the at least two signals may be received as an
orthogonal frequency division multiplexed signal conforming to a
communications protocol, at least one of the signals may be
modified to generate the at least two alternate representations,
each of which can be demodulated by a receiver compatible with the
protocol without requiring receipt of additional information
outside of the communications protocol, and the at least one
criterion may comprise a peak to average power ratio of the
combined signal. The at least one criterion may comprise a peak to
average power ratio of the combined signal. The selected combined
signal may be a combined signal representing a lowest peak to
average power ratio, or a peak to average power ratio within a
threshold level.
[0224] The MSM signal may be an orthogonal frequency multiplexed
(OFDM) signal having a plurality of subcarriers at different
frequencies which concurrently communicate the information. The MSM
signal and the other signal may each be within a different
communication channel and be processed as a combined analog signal
in at least one analog signal processing component having a
non-linear distortion. The MSM signal may comply with a
predetermined protocol which selectively inserts the pilot signals
in a plurality of the subcarriers at different times and at
different frequencies, to estimate the channel state. The at least
one parameter may comprise a cyclic shift of digital data
representing the MSM signal, wherein the model predicts an ability
of a receiver which complies with an OFDM protocol to detect the
pilot and estimate the channel state subject to at least two
different cyclic shifts. The at least one fitness criterion may be
dependent on the non-linear distortion of the combination of the
MSM signal with the other signal in the analog signal processing
component.
[0225] It is a further object to provide an apparatus for
controlling a combined waveform, representing a combination of at
least two signals having a plurality of signal components,
comprising: an input port configured to receive information
defining the at least two signals; a processor configured to:
transform the information defining each signal into a
representation having a plurality of components, such that at least
one signal has alternate representations of the same information
along with further information to permit a receiver to estimate a
channel state, and combining the transformed information using the
alternate representations, to define at least one combination which
meets a predetermined criterion and which permits the receiver to
estimate the channel state, wherein the transform is adapted to
define at least one alternate representation which fails to permit
the receiver to estimate the channel state; and an output port
configured to output information representing a respective combined
waveform comprising the combination of the transformed information
from each of the at least two signals which meets the
criterion.
[0226] Thus, the transmitter may combine signals in such a way that
the combination may violate a first criterion, but that the same
information may be combined by altering the combination without
violating the first criterion. However, the altered combination may
violate a second criterion, that the un-altered combination
generally does not violate. The processor seeks to find an
alteration, which may require a search through a range, which meets
both the first criterion and the second criterion. The second
criterion relates to a communication from the transmitter to the
receiver of pilot information which calibrates the receiver and for
example permits the receiver to estimate the channel state. The
pilot signals may be sparsely inserted into the combined signal,
and the receiver may estimate the channel state based on a series
of communicated pilot signals over a series of data frames.
[0227] A first combination and a second combination of the
transformed information may differ with respect to at least one of
(a) a relative timing of a modulation of the frequency components
of a first signal with respect to a second signal and (b) a
relative phase of the frequency components of a signal, and the at
least one criterion may comprise a peak to average power ratio
(PAPR). At least one signal may be an orthogonal frequency division
multiplexed stream which is compatible with at least one protocol
selected from the group of an IEEE 802.11 protocol, an IEEE 802.16
protocol, a 3GPP-LTE downlink protocol, a DAB protocol and a DVB
protocol, wherein a receiver compliant with the at least one
protocol can demodulate the at least two respectively different
combinations without requiring additional information to be
transmitted outside of the protocol. Each transformed
representation may differ respectively in a cyclic time shift in a
modulation sequence. The at least two criteria comprise a peak to
average power ratio (PAPR), wherein an alternate representation
which results in a peak to average power ratio within a threshold
maximum peak to average power ratio is selected for
combination.
[0228] The at least two alternate representations may differ
respectively in a cyclic time shift in a modulation sequence, and
the at least one criterion may comprise a peak to average power
ratio (PAPR), wherein an alternate representation which results in
a lowest peak to average power ratio is selected for
combination.
[0229] The processor may comprise at least one of superconducting
digital circuit logic and a complex programmable logic device
(CPLD).
[0230] The output port may be configured to output the selected
combined signal as a direct conversion from a digital
representation of the combined signal to a radio frequency analog
signal adapted for transmission without frequency modification.
[0231] The processor may be further configured to predistort at
least one of an intermediate frequency and a radio frequency
representation of the selected combined signal.
[0232] The predistortion may compensate for at least a portion of
one or more of an analog non-linearity, a transmission channel
impairment, and a receiver characteristic of an analog radio
communication system communicating using the selected combined
signal.
[0233] The signals may comprise orthogonal frequency division
multiplexed signals. A first combination and a second combination
of the transformed information may differ with respect to at least
one of (a) a relative timing of a modulation of the frequency
components of a first signal with respect to a second signal and
(b) a relative phase of the frequency components of a signal. The
at least one criterion may comprise a peak to average power ratio
(PAPR). The selected combination may comprise a combination with a
lowest peak to average power ratio.
[0234] Each transformed representation may differ respectively in a
cyclic time shift in a modulation sequence, and wherein the
orthogonal frequency division multiplexed signals are compatible
with at least one protocol selected from the group of an IEEE
802.11 protocol, an IEEE 802.16 protocol, a 3GPP-LTE downlink
protocol, a DAB protocol and a DVB protocol, wherein a receiver
compliant with the at least one protocol can demodulate the at
least two respectively different combinations of without requiring
additional information to be transmitted outside of the protocol,
each transformed representation differing respectively in a cyclic
time shift in a modulation sequence, and the at least one criterion
may comprise a peak to average power ratio (PAPR), wherein an
alternate representation which results in a lowest peak to average
power ratio is selected for combination. The processor may comprise
at least one of superconducting digital circuit logic and a complex
programmable logic device (CPLD). The processor may analyze a
nonlinear distortion of the combined waveform in a model of an
amplifier, and further predistort at least of at least one
component of the selected combination and the selected
combination.
[0235] Further objects will become apparent through a review of the
detailed description and figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0236] FIGS. 1A and 1B show typical behavior of an orthogonal
frequency-domain multiplexed channel in the frequency and time
domains, respectively.
[0237] FIG. 2 represents a time-frequency resource grid for an OFDM
channel, showing typical locations of pilot symbols according to
the protocol for LTE.
[0238] FIG. 3 shows the combination of two OFDM channels in a
transmitter using a double-upconversion method.
[0239] FIG. 4 provides a simple block diagram showing how two OFDM
channels may be combined, wherein the data bits of one OFDM channel
may be cyclically shifted in order to reduce the peak-to-average
power ratio (PAPR).
[0240] FIG. 5 shows a block diagram of an OFDM communication system
that incorporates the shift-and-add algorithm in the transmitter
and a pilot phase tracker in the receiver.
[0241] FIG. 6 shows a block diagram of an OFDM communication system
that enables the SAA algorithm for a resource grid as in FIG.
2.
[0242] FIG. 7 shows a top-level flowchart for a generalized carrier
aggregation method of the present invention.
[0243] FIG. 8 shows a block diagram of an OFDM communication
system, whereby the receiver generates an equalized resource grid
based on an array of pilot symbols as in FIG. 2.
[0244] FIG. 9 shows a block diagram that represents the process of
equalizing the resource grid at the receiver using an array of
pilot symbols as in FIG. 2.
[0245] FIG. 10 shows the structure of two OFDM channels, with
cyclic shifting of the data for one channel in order to reduce the
PAPR of the combined signal.
[0246] FIG. 11 provides a block diagram showing memory storage of
multiple shifted replicas of data from an OFDM channel, with
selection of one replica corresponding to minimizing the PAPR of
the combined signal.
[0247] FIG. 12A shows a typical 64QAM constellation diagram for a
simulated OFDM received signal without added noise.
[0248] FIG. 12B shows a 64QAM constellation diagram for a simulated
OFDM received signal with noise added.
[0249] FIG. 13A shows a probability plot for PAPR of the carrier
aggregation of simulated OFDM signals, showing reduced PAPR for the
method of the invention.
[0250] FIG. 13B shows a probability plot for PAPR of the carrier
aggregation of simulated OFDM signals, including the effect of
digital predistortion.
[0251] FIG. 13C shows a block diagram of the simulation with
results shown in FIGS. 13A and 13B.
[0252] FIG. 14 shows a block diagram of a system according to one
embodiment of the invention.
[0253] FIG. 15 shows a block diagram of a system according to one
embodiment of the invention, including digital predistortion to
compensate for a nonlinear analog amplifier.
[0254] FIG. 16 shows a block diagram of a system according to the
invention, implemented on an FPGA.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0255] OFDM channels are comprised of many sub-channels, each of
which is a narrow-band signal (FIG. 1A). An OFDM channel itself has
a time-varying envelope, and may exhibit a substantial PAPR,
typically 9-10 dB. However, if two separate similar OFDM channels
are combined, the resulting signal will typically exhibit PAPR of
12-13 dB, for a gain of 3 dB. This is unacceptably large, since it
would require a power amplifier with 4 times the capacity to
transmit a combined signal that averages only 2 times larger.
[0256] A preferred embodiment therefore provides a PAPR reduction
method which reduces the PAPR of a two OFDM channel combined signal
from 12-13 dB back down to the 9-10 dB of the original components.
This .about.3 dB reduction in PAPR is preferably accomplished
without degradation of the signal, and without the need to transmit
any special side information that the receiver would need to
recover the OFDM symbols. Further, the algorithm is simple enough
that it can be implemented in any hardware technology, as long as
it is sufficiently fast.
[0257] Conventional methods of PAPR reduction focus on combining
the sub-channels and generating a single OFDM channel without
excessive PAPR. The present technique can be viewed in certain
respects as a combination of Partial Transmit Sequence (PTM) and
Selected Mapping (SLM).
[0258] In traditional PTS, an input data block of N symbols is
partitioned into disjoint sub-blocks. The sub-carriers in each
sub-block are weighted by a phase factor for that sub-block. The
phase factors are selected such that the PAPR of the combined
signal is minimized.
[0259] In the SLM technique, the transmitter generates a set of
sufficiently different candidate data blocks, all representing the
same information as the original data block, and selects the most
favorable for transmission (lowest PAPR without signal
degradation).
[0260] The present hybrid approach combines elements of PTS and SLM
for summed carrier modulated signals. The various cyclic
time-shifts of the oversampled OFDM waveform are searched, and the
time-shift with the lowest PAPR selected. One OFDM signal is used
as reference and the other carrier modulated signal(s) are used to
generate the time-shifts, in a manner similar to PTS. The search
window is determined by the cyclic prefix length and the
oversampling rate.
[0261] While the phase space of possible combinations of shifts
increases tremendously, it would not be necessary to explore all
such combinations. In general, very high values of the PAPR occur
relatively rarely, so that most time shifts starting with a
high-PAPR state would tend to result in a reduction in PAPR. Shifts
in multiple channels could be implemented sequentially or in
parallel, or in some combination of the two. Thus, for example, any
combination with a PAPR within an acceptable range is acceptable,
any unacceptable PAPR states occur 1% of the time, the search space
to find an acceptable PAPR would generally be <2% of the
possible states. On the other hand, if other acceptability criteria
are employed, a larger search space may be necessary or
appropriate. For example, assuming that there is a higher cost for
transmitting a higher PAPR signal, e.g., a power cost or an
interference cost, then a formal optimization may be appropriate.
Assuming that no heuristic is available for predicting an optimal
state, a full search of the parametric space may then be
appropriate to minimize the cost.
[0262] This differs from conventional approaches, wherein different
OFDM channels are independent of one another, with separate
transmit chains and without mutual synchronization. Further, the
conventional approaches operate directly on the baseband signals.
In contrast, the present method evaluates PAPR on an up-converted,
combined signal that incorporates two or more OFDM channels, and
the symbol periods for each of these channels must be synchronized.
This will not cause problems at the receivers, where each channel
is received and clocked independently.
[0263] Some conventional approaches to PAPR are based on clipping,
but these inevitably produce distortion and out-of-band generation.
Some other approaches avoid distortion, but require special
transformations that must be decoded at the receive end. These
either require sending side-information, or involve deviations from
the standard OFDM communication protocols. The present preferred
approach has neither shortcoming.
[0264] OFDM channels used in cellular communications, may be up to
10 or 20 MHz in bandwidth. However, these channels might be located
in a much broader frequency band, such as 2.5-2.7 GHz. So one might
have a combination of two or more OFDM channels, each 10 MHz wide,
separated by 100 MHz or more. A 10 MHz digital baseband signal may
be sampled at a rate as low as 20 MS/s, but a combined digital
signal covering 100 MHz must be sampled at a rate of at least 200
MS/s.
[0265] In a preferred embodiment, the signal combination (including
the up-conversion in FIG. 3) is carried out in the digital domain
at such an enhanced sampling rate. The PAPR threshold test and CSR
control are also implemented at the higher rate. This rate should
be fast enough so that multiple iterations can be carried out
within a single symbol time (several microseconds).
[0266] In order to verify the expectation that the circular
time-shift permits reduction in PAPR for combined OFDM channels,
without degrading system performance, a full Monte-Carlo simulation
of OFDM transmission and reception was carried out. The block
diagram of this simulation is summarized in FIG. 6, which
represents the "Carrier Aggregation Evaluation Test Bench", and
shows a transmitter that combines OFDM signals at frequencies
F.sub.1 and F.sub.2, subject to the SAA algorithm for PAPR
reduction. At the receive end, this is down-converted and the
signal at F.sub.2 is recovered using a standard OFDM receiver.
Along the way, appropriate Additive White Gaussian Noise (AWGN) is
added to the channel. The parameters for the Carrier Aggregation
simulations include the following. Each packet contains 800 bytes
of information, which is modulated over several OFDM symbol
periods, and the modulation is 64-QAM (64-quadrature amplitude
modulation). Each SNR point is run until 250 packet errors occur.
The cyclic prefix is set to 1/8 of the total symbol time. Carriers
at frequencies F.sub.1 and F.sub.2 are spaced sufficiently that
their spectra do not overlap. The oversampling rate is a factor of
8. Finally, a raised cosine filter was used, with a very sharp
rolloff, with a sampling frequency F.sub.s=160 MHz, and a frequency
cutoff F.sub.c=24 MHz. FIG. 12A shows an example of a constellation
chart of the 64-QAM received signals for the simulation without
noise, where a time shift has been applied that is expected to be
compatible with the interpolation equalizer of the receiver. In
this example, no pilot symbol was transmitted during this time
period. The clustering indicates that each bit is received within
its required window, with no evidence of bit errors. More
generally, no degradation of the signal was observed for an
allowable time shift, as expected. FIG. 12B shows a similar 64-QAM
constellation chart for the simulation with added Gaussian noise
typical of a practical wireless communication system. Again, the
simulation shows proper reception of the signal with no significant
increase in bit errors.
[0267] FIG. 13A shows the simulated PAPR distribution for a
combination of two OFDM signals, combined according to an
embodiment of the invention. The Complementary Cumulative
Distribution Function (CCDF) represents the probability that the
signal has a PAPR greater than a given value. For practical
purposes, a CCDF of 10.sup.-4 can be used to define the effective
PAPR of a particular waveform. Each of the two component signals
has a PAPR of 11 dB (top curve). The combination of the two signals
without modification would lead to an increase in PAPR of almost 3
dB (not shown). In contrast, combination using the Codebook
Pre-Weighting algorithm of the present invention leads to a
decrease of almost 2 dB to about 9 dB (bottom curve). This benefit
would be reduced if this Codebook approach is not applied (middle
curve).
[0268] FIG. 13B shows the effect of applying digital predistortion
(DPD) in addition to Crest Factor Reduction (CFR), as indicated in
the simulation block diagram of FIG. 13C. FIG. 13C shows the
combination of three OFDM signals, each corresponding to LTE
signals of 20 MHz bandwidth. The individual baseband signals are
sampled at 30.72 MHz, followed by upsampling to 122.8 MHz,
offsetting the frequencies (using a digital multiplier), and adding
together to form an IF signal with a 60 MHz band comprising three
20 MHz bands. This is then subject to Crest Factor Reduction (CFR)
according to the Codebook Weighting algorithm of the present
invention, followed by upsampling (by a factor of two) and digital
predistortion (DPD, to simulate the saturation effect of a
nonlinear power amplifier PA). Finally, the predistorted signal is
sent to a digital-to-analog converter (DAC) and then amplified in
the PA. The curve in FIG. 13B labeled CFR Input shows the combined
signal, while CFR Output shows the result of PAPR reduction. The
curve labeled "+n SCA Processing" (PlusN Smart Carrier Aggregation)
corresponds to the signal as broadcast, including the effects of
predistortion.
[0269] These simulations have confirmed not only that the SAA
algorithm permits reduction of PAPR in combined OFDM channels by
.about.3 dB, but also that this reduction is achieved without
signal degradation and without the need to send any special side
information on the transformations in the transmit signal. This can
also be integrated with digital predistortion, without degradation
of the PAPR reduction.
[0270] A block diagram of a system according to one embodiment of
the invention is shown in FIG. 14, where at least one of the input
signals is identified as a multiple subchannel multiplexed (MSM)
signal, essentially a generalization of an OFDM signal. The MSM
signal is assumed to include pilot signals independent of the
information content, which enable the signal to be properly
received in the presence of multi-path, Doppler shift, and noise.
Here the MSM signal and another signal are combined in a plurality
of alternative aggregated signals, where each such alternative
combination could be properly received for both such signals at a
receiver, without sending additional side-information. A digital
model of the receiver, which may incorporate prior transmitted
signals, permits determination of which alternative combinations
correspond to MSM pilot signals that can be properly tracked at the
receiver. Based on this criterion, and at least one other criterion
that may be associated with combined signal amplitude (such as
peak-to-average-power ratio or PAPR), one or more of the
alternative combinations is selected, which may be subject to
further processing or selection, e.g., in an iterative selection
process using various criteria, for transmission using an automated
processor. This may preferably be carried out using a digital IF
signal, which is then converted to an analog signal in a
digital-to-analog converter (DAC), and then upconverted to the full
radio frequency signal in the standard way in an analog mixer
before being amplified in the Power Amplifier and transmitted via
an antenna. Other types of RF modulators may also be employed.
[0271] FIG. 15 represents a block diagram similar to that in FIG.
14, but with the addition of digital predistortion modules that
compensate for nonlinearities that may be present in nonlinear
analog components such as the Power Amplifier (including inherent
non-linearities, signal-dependent delays, saturation and heating
effects). The predistortion is preferably carried out on the
alternative combinations, so that the selected combination(s)
properly meet all criteria.
[0272] The predistortion may encompass correction of multiple
distortion sources, and represent transformations of the signal in
the time (delay) and/or frequency domains, amplitude and waveform
adjustments, and may be adaptive, for example, to compensate for
aging and environmental conditions. In the case of multiple-input
multiple-output (MIMO) radio transmission systems (or other signal
transmissions), the distortion model encompasses the entire signal
transmission chain. This model may include distinct models for the
various multipaths, and therefore the selected alternative
predistorted signal may represent an optimum for the aggregate
system, and not only the "principal" signal component.
[0273] One preferred implementation of the technique involves using
a fast field-programmable gate array (FPGA) with blocks for
shift-register memories, lookup tables, digital up-conversion, and
threshold testing. This is illustrated in FIG. 16, which also shows
the optional addition of digital predistortion. In this embodiment,
the input digital baseband signals (in the time domain) are first
stored in memory registers within the FPGA, and the MSM signal S2
is transformed in a plurality of digital precoders. In one
embodiment, these precoders may comprise circular shift registers
(CSRs) with different values of the shift parameter. In other
embodiments, the range of parameter variation is not time (i.e.,
the incremental variation in a CSR), but rather another parameter,
such as the time-frequency range of a wavelet transform. These
shifted versions are chosen so as to be compatible with pilot
signal tracking in the receiver, as determined by the lookup-table
discriminator block. This LUT may take into account prior shifts,
as shown in FIG. 16. FIG. 16 shows several precoding schemas (e.g.,
circular shifts) being processed in parallel, although serial
processing is also possible. Each baseband signal to be combined is
subjected to digital upconversion in the digital upconverter DUC to
a proper intermediate frequency (IF), with an increase in sampling
rate as appropriate. Sample S1 and each alternative S2 may then be
digitally combined in the Digital IF Combiner unit. This is
followed by optional digital predistortion in digital predistorters
PD, before each alternative combination is sent to the Threshold
Tester. The Threshold Tester may, for example, measure the PAPR of
each alternative, and choose the alternative with the lowest
PAPR.
[0274] Alternatively, an ultrafast digital technology, such as
rapid-single-flux-quantum (RSFQ) superconducting circuits, may be
employed. As the number of OFDM channels being combined is
increased, one needs either to increase the algorithm speed, or
alternatively carry out a portion of the processing in
parallel.
[0275] This method may also be applied to a reconfigurable system
along the lines of cognitive radio, wherein the channels to be
transmitted may be dynamically reassigned depending on user demand
and available bandwidth. Both the number of transmitted channels
and their frequency allocation may be varied, under full software
control. As long as all channels follow the same general symbol
protocol and timing, one may apply a similar set of Shift-and-Add
algorithms to maintain an acceptable PAPR for efficient
transmission.
* * * * *
References